The present disclosure relates to a photoelectric conversion apparatus and a radiographic imaging apparatus, and particularly to a radiographic imaging apparatus or radiographic reading apparatus which wavelength converts radioactive rays represented by α-rays, β-rays, γ-rays and X-rays into rays in a sensitive range of a photoelectric conversion apparatus with a wavelength conversion member to read information based on the radioactive rays.
In a photoelectric conversion apparatus and a radiographic imaging apparatus, charge produced by photoelectric conversion by a photoelectric conversion section based on input information is transferred to an external capacitor, by which the charge is converted into a signal voltage. By transferring the charge from the capacitor of the photoelectric conversion section itself to the external capacitor to convert the charge into a signal voltage in this manner, a comparatively high S/N ratio can be obtained.
Incidentally, where a configuration is applied wherein a plurality of pixels are arranged in a juxtaposed relationship with each other, the wiring line length of a signal line for reading out a signal from a pixel becomes great in accordance with the number of the pixels, and parasitic capacitance is sometimes formed. For example, it is assumed that a great number of pixels individually having the size of 200 μm×200 μm are arranged in a matrix of 2,000 pixels in the vertical direction×2,000 pixels in the horizontal direction to produce an area sensor having a size equal to that of an X-ray film, for example, the size of 40 cm×40 cm.
Where an area sensor has a size equal to that of an X-ray film, capacitance is formed by an overlap of the source region and the gate electrode of a transistor for charge transfer. Since a number of overlaps equal to the number of pixels are formed, even if the overlap capacitance Cgs is approximately 0.05 pF per one place, a capacitance of 0.05 pF×2,000=100 pF is formed on one signal line.
Since the capacitance Cs of the photoelectric conversion section itself, that is, the sensor capacitance, is approximately 1 pF, where a signal voltage generated in a pixel is represented by V1, the output voltage V0 of a signal line is given byV0={Cs/(Cs+Cgs×1000)}×V1and the output voltage becomes approximately 1/100. In other words, the output voltage drastically decreases where an area sensor having a great area is configured.
Further, in order to carry out reading of a dynamic picture image in such a situation as described above, a sensitivity and a high-speed performance of operation with which image reading of 30 or more images per one second are required. Particularly, also it is demanded to minimize the dose of X-rays to be irradiated in a non-destructive inspection including X-ray diagnosis in the medical care, and it is demanded to further enhance the sensitivity such that the signal charge amount can be increased to 100 to 400 times.
On the other hand, a photoelectric conversion apparatus which is configured such that a source follower circuit is provided for each pixel is known and disclosed, for example, in Japanese Patent Laid-Open No. Hei 11-307756 (refer, particularly to paragraphs 0040 to 0044 and FIG. 7, hereinafter referred to as Patent Document 1). The source follower circuit includes a field-effect transistor for receiving signal charge generated by a photoelectric conversion section at the gate thereof to read out a signal voltage corresponding to the signal charge into a signal line. The source follower circuit makes it possible to read out a signal at a high speed also where the capacitance formed on the signal line is high.
An example of a pixel structure in related art is shown in FIG. 10. Referring to FIG. 10, a pixel 100 shown includes a driving device section including a transistor 101 having a bottom gate structure and a PIN (Positive Intrinsic Negative Diode) photodiode 102. The PIN photodiode 102 has a structure wherein an n-type semiconductor layer 103, an i-type semiconductor layer 104 and a p-type semiconductor layer 105 are laminated in order and patterned into a substantially same shape. The i-type semiconductor layer 104 is formed with a thickness of approximately 1 μm, for example, from amorphous silicon.
The photoelectric conversion apparatus disclosed in Patent Document 1 is structured such that the n-type semiconductor layer 103 and the p-type semiconductor layer 105 have a substantially same shape. Therefore, the edge portions of the both semiconductor layers 103 and 105 are disposed very closely to each other sandwiching the i-type semiconductor layer 104 having a thickness of at most approximately 1 μm therebetween. Therefore, leakage current is likely to be generated between the edges of the n-type semiconductor layer 103 and the p-type semiconductor layer 105 on the interface of them with an inter-layer insulating film 106.
The photoelectric conversion apparatus in related art has a problem that, since a photoelectric conversion device in which leakage current is generated, for example, the PIN photodiode 102, is disabled to normally accumulate photoelectrically converted charge, the photoelectric conversion device becomes a defective device. Even if the photoelectric conversion device does not become a defective device, if weak leakage current flows, then the leakage current makes a factor of dispersion of a device characteristic. Therefore, accurate photoelectric conversion or image pickup in accordance with incident light or incident energy cannot be achieved.
Therefore, it is desirable to provide a photoelectric conversion apparatus which can suppress leakage current between edges of semiconductor layers of the opposite conductivity types in a photoelectric conversion device and a radiographic imaging apparatus in which the photoelectric conversion apparatus is used.